Solid organ transplantation is the therapy of choice for end-stage diseases, but the ensuing life-long use of immunosuppressive drugs compromises the quality of life and overall survival of these patients. The development of a treatment that minimizes the number of interventions, and associated side effects, would be transformative for this field. Clinical testing of the modulation of graft rejecting T lymphocytes through costimulation blockade (use of the biologic CTLA4-Ig, recently approved by the FDA) has shown improvements over the side effects of conventional immunosuppression, but has also shown unacceptably high rates of acute rejection episodes. This indicates that the initial view of activation of the rejection response was oversimplified and additional factors contribute to its modulation. Our ongoing experiments support the emerging theory that inflammatory cytokines (released during and after transplantation) neutralize the modulatory effect of CTLA4-Ig. We have discovered that short-term inhibition of the production and signaling of inflammatory cytokines, through the small molecule inhibitor Tofacitinib (Tofa), synergizes with CTLA4-Ig to prevent the activation of T cells and promotes transplant survival in a mouse model of heart transplantation. Due to the short in vivo half-life of Tofa, and the side effects associated with is chronic use, the clinical translatability of our approach hinges on realizing precise control over Tofa release and biodistribution, so to maximize its therapeutic effect. To achieve this, we propose to use an emerging class of nanoparticles for drug delivery: lipid-derived nanoparticles (LNp). In addition to their proven advantages with respect to biocompatibility and stability, LNp also have the unique ability to accumulate in the lymphatic system following administration and, if properly sized, they can cross biological barriers like skin (as indicated in our preliminary daa) or the intestinal membrane. We propose the hypothesis that LNp-mediated in vivo delivery of Tofa will result in transient, but controlled, localized modulation of the immune environment of a transplant recipient and will maximize the modulatory effect of CTLA4-Ig, realizing what we define Enhanced Costimulation Blockade (ECoB) and promoting long-term graft survival. This hypothesis will be addressed in the experiments of two Specific Aims: (1) to optimize the formulation of Tofa-LNp to maximize their impact on innate and adaptive immune cells responsible for rejection; and (2) to define the route of administration for optimal in vivo distribution of Tofa- LNp to achieve ECoB-mediated modulation of heart graft alloreactivity and promote long-term survival. Overall, these studies are designed to define the foundation for the development of a platform of intervention (LNp-based) for localized and safe immune-modulation that could be exploited to transform the treatment of transplant patients as well as further evolved to benefit patients with immune pathologies.